The study describes aluminium-silicon protective coatings produced by the slurry cementation process on a substrate of creep-resistant Ni-Cr cast steel. The mechanism of the coating growth on a substrate was examined and the effect of annealing time and temperature on total thickness of the coatings and on the thickness of the individual zones was analyzed. The coating structure is composed of two zones. In the external zone, elements like Al, Ni and Fe are concentrated, forming intermetallic phases of an Al(Ni,Fe) type, while the internal zone contains an (Fe,Cr,Si) solution. The internal zone formed in a sub-surface layer of the substrate has hardness values intermediate between the hardness of the substrate and that of the external layer.